The invention relates generally to an interferometer system for position measurement, and more specifically, an interferometer system and method for improving the accuracy of interferometric measurements.
A laser interferometer is often used to accurately measure relative displacement between two members in a projection exposure system used to manufacture semiconductor devices. The laser interferometer is used as a measuring apparatus for measuring the coordinates of a wafer stage or mask stage for highly accurate positioning of a semiconductor wafer or reticle relative to stationary projection optics. A prior art laser interferometer system is shown in FIG. 1. The interferometer system typically measures a change in position of a measurement mirror Mx, My (not shown in FIG. 1) attached to a movable stage S relative to a stationary reference mirror R attached to a projection lens PL. A laser source generates a beam B of light, part of which is reflected from the reference mirror R and part of which is reflected from the measurement mirror Mx (FIG. 1). The light reflected from the two mirrors Mx, R is then combined and reflected into a sensor SR. If the measurement mirror Mx moves relative to the reference mirror R, the intensity of the combined beam periodically increases and decreases as the reflected light from the two paths alternately interferes constructively and destructively. This constructive and destructive interference is caused by the two beams moving in and out of phase. Each half wavelength of movement of the measurement mirror results in a total optical path change of one wavelength and thus, one complete cycle of intensity change. The number of cycle changes indicates the number of wavelengths that the measurement mirror has moved. Therefore, by counting the number of times the intensity of the light cycles between darkest and lightest, the change in position of the measurement mirror can be estimated as an integral number of wavelengths.
As manufacturers of integrated circuits attempt to increase circuit density and reduce circuit feature size, interferometers are required to provide more precise measurement data. The precision with which interferometers provide position control has been improved by technical advances in the design of various optical components including lasers and photosensors. However, the performance of interferometers is still limited by changes in optical path length due to environmental disturbances that cause movement of the optical components of the interferometer system. When optical components, such as a beam splitter BS tilts or rotates due to thermal fluctuations, for example, the distance between the reference mirror R and the beam splitter changes (FIG. 1). This movement of the beam splitter BS causes an error in the position measurement of the stage S that results in misalignment of circuit patterns on the wafer W relative to one another.
There is, therefore, a need for an interferometer system that measures and corrects for movement of the optical components of the interferometer system.
The invention overcomes deficiencies of the prior art by providing an interferometric measuring system that measures and corrects for movement of optical components within the measuring system to provide a more accurate position measurement of a moving member such as a stage.
The interferometric measuring system generally includes a movable member having a measurement reflector attached thereto, a stationary member having a reference reflector attached thereto, a light source for providing a light beam, and an optical support block having first and second beam splitters mounted thereon. The first beam splitter is operable to convert the light beam into a first pair of polarized beams and arranged to direct one of the first pair of beams along a reference path to the reference reflector and the other of the first pair of beams along a measurement path to the measurement reflector to create a measurement interference pattern between the first pair of beams for determining a position of the movable member. The second beam splitter is operable to form a second pair of polarized beams and arranged to direct the second pair of beams along support block measurement paths to the reference reflector. The second beam splitter is configured to form a correction interference pattern between the second pair of beams for determining error in the position of the movable member due to movement of the optical support block.
In one embodiment, the beam splitter is optically aligned with the measurement reflector and interposed between the light source and the measurement reflector. Additional beam splitters and right angle reflectors may be included in the system to direct the beams along their respective paths.
A method of the invention is for measuring a position of a movable member relative to a stationary member with an interferometric measuring system and correcting for movement of optical components within the system relative to the stationary member. The movable member has a measurement reflector attached thereto and the stationary member has a reference reflector attached thereto. The method generally includes sensing a measurement interference pattern between a measurement light beam propagated through a measurement optical path including the measurement reflector, and a reference light beam propagated through a reference optical path including the reference reflector to determine a position of the movable member relative to the stationary member; and sensing a correction interference pattern between two correction light beams propagated through optical component measurement paths to determine movement of the optical components relative to the stationary member. The position of the movable member is then corrected to compensate for movement of the optical components.
The above is a brief description of some deficiencies in the prior art and advantages of the invention. Other features, advantages, and embodiments of the invention will be apparent to those skilled in the art from the following description, drawings, and claims.